1. Field of the Invention
The present invention relates generally to external defibrillators. In particular, the present invention is an automatic external defibrillator for generating and testing biphasic waveforms.
2. Description of the Related Art
Cardiac arrest, exposure to high voltage power lines and other trauma to the body can result in heart fibrillation, the rapid and uncoordinated contraction of the cardiac muscle. The use of external defibrillators to restore the heartbeat to its normal pace through the application of an electrical shock is a well recognized and important tool for resuscitating the patients.
Commercially available defibrillators such as those available from Surviva Link Corporation, the assignee of the present application, are configured to produce monophasic waveform defibrillation pulses. Monophasic (i.e., single polarity) pulses such as the Lown waveform and the truncated exponential waveform have been demonstrated to be effective for defibrillation, and meet standards promulgated by the Association for Advancement of Medical Instrumentation (AAMI). Electrical circuits for producing monophasic waveform defibrillation pulses are generally known and disclosed, for example, in the Persson U.S. Pat. No. 5,405,361.
The efficacy of biphasic waveform pulses (actually two successive pulses of opposite polarities) has been established for implantable defibrillators. For example, studies conducted on implantable defibrillators have shown that biphasic waveform defibrillation pulses result in a lower defibrillation threshold than monophasic pulses. A variety of theories have been proposed to explain the defibrillation characteristics of biphasic waveform pulses.
It is anticipated that the efficacy and advantages of biphasic waveform pulses that have been demonstrated in implantable defibrillators will be demonstrated in external defibrillators as well. However, because of the difficulties involved in testing external defibrillators, the efficacy of biphasic waveform pulses produced by these instruments has not yet been fully studied. External defibrillation is typically used in emergency settings in which the patient is either unconscious or otherwise unable to communicate. It is therefor difficult if not impossible to obtain informed consent from the patient for such tests. Time is also of the essence since studies have shown that the chances for successful resuscitation diminish approximately ten percent per minute. The use of experimental defibrillation pulses must therefor be completed quickly. The capability of quickly switching to a proven defibrillation pulse following an unsuccessful rescue with the experimental pulse must also exist. For these reasons it is evident that there is a need for external defibrillators and protocols capable of producing and testing biphasic waveform pulses.